Electrochemical oxygen sensors are generally known. Such sensors typically rely upon a redox reaction in first and second parts of the sensor. In this case, a precious metal cathode in a first part of the sensor chemically reduces oxygen while a balancing reaction oxidizes a consumable anode (e.g., lead) in a second part of the sensor.
The cathode and consumable anode are coupled through the use of an ionically conducting electrolyte. The second part of the sensor may contain or be filled with the electrolyte. The anode is saturated with this electrolyte.
A fiber separator may separate the first and second parts of the sensor. The fiber separator bounds the second part of the sensor and also becomes saturated with the electrolyte. The separator also contacts the cathode and supports the ionic transfer between the cathode and anode.
During use, oxygen diffuses into the first part of the sensor through an aperture and a gas phase diffusion barrier to react with the cathode. The aperture (capillary) is usually the diffusion controlling element in the design. The membrane shown in the types of sensors considered here is the supporting element for the sensing electrode and is designed not to offer a large diffusion resistance. In this way the sensor performance is controlled by well understood properties of the mechanical capillary rather than the more complex and variable properties of a tape. There is a different style of sensor which uses a solid membrane as the diffusion barrier, through which gas percolates via a form of solid solution process but this has a different type of pressure response.
Vents are more widely known and used for pressure relief in fuel-cell type electrochemical sensors where the drawbacks associated with parasitic consumption of the consumable component is not an issue as is the case for oxygen. In fact, the early attempts to obtain patent coverage on vented oxygen sensors were limited in their technological scope for this reason.
While electrochemical oxygen sensors work well, their operation can become degraded over time. For example, the separator may leak allowing the bulk transfer of gas between the first and second parts of the sensor. In cases where the sensor is subjected to temperature changes, expansion or contraction of gas within the sensor may produce pressure gradients across the separator which can result in bubbles of gas being forced through the separator. When this occurs, gas needs to flow through the capillary of the sensor to compensate for the change in volume due to movement of the bubble(s), which is inconsistent with the principle of diffusion under which the sensor operates. The bulk transport of gas through the first part of the sensor causes the sensor to produce erroneous readings through a process commonly referred to as “glitching.” Because of the importance of electrochemical gas sensors, a need exists for methods of providing more reliable sensors.